Proton transfer energy profile

Outer sphere electron transfer (e.g., [11-19,107,160-162]), ion transfer [10,109,163,164] and proton transfer [165] are among the reactions near electrodes and the hquid/liquid interface which have been studied by computer simulation. Much of this work has been reviewed recently [64,111,125,126] and will not be repeated here. All studies involve the calculation of a free energy profile as a function of a spatial or a collective solvent coordinate. 

Any doubt about the existence of individual tautomers is now long past some tautomers can be crystallized separately (desmotropy), and others can be observed simultaneously in the same crystal (Section V,D,2) in summary, tautomers are not intrinsically different from isomers. Maybe it is worth mentioning that even two identical tautomers can differ. This is the case for the two intramolecular hydrogen-bonded (IMHB) enol tautomers of acetylacetone and for many NH-azoles they correspond to a doublewell profile for the proton transfer with both wells having the same energy (autotrope). 

Figure 13-4. Energy profile for the proton transfer in malonaldehyde enol. 

Fig. 1 Free energy reaction coordinate profiles for hydration and isomerization of the alkene [2] through the simple tertiary carbocation [1+], The rate constants for partitioning of [1 ] to form [l]-OSolv and [3] are limited by solvent reorganization (ks = kteorg) and proton transfer (kp), respectively. For simplicity, the solvent reorganization step is not shown for the conversion of [1+] to [3], but the barrier for this step is smaller than the chemical barrier to deprotonation of [1 ] (kTtOTg > kp).

PROTON TRANSFER REACTION PATHWAY 4.1 Free Energy Profile... 

Figure 8. Total free energy profile for the transfer of the proton as a function of H04...N5 distance.

Table 4. Ab initio energy profile during the proton transfer starting from the reduced tetrahydrofolate. ...

In this study, identification of the critical atomic and molecular determinants pertaining to the mechanism of dihydrofolate to tetrahydrofolate reduction was achieved by (i) ab initio quantum mechanics, (ii) molecular mechanics, and (iii) free energy perturbation techniques. For the first time, the complete free energy profile was calculated for the proton transfer from Asp27 of the enzyme E. Coli DHFR to the N5 position of the dihydropterin moiety of the substrate dihydrofolate. In addition, the free... 

A structure-structure correlation may itself contain some of the necessary information. Note that in Fig. 4 the points are most abundant in the regions where dx and d2 are about 1.0 and 1.5-2 A, respectively, and sparse close to the point where dt and d2 are equal. This distribution is expected if the symmetrical system is of higher energy, so that the energy profile diagram for the proton transfer reaction (5)... 

Ah initio calculations to map out the gas-phase activation free energy profiles of the reactions of trimethyl phosphate (TMP) (246) with three nucleophiles, HO, MeO and F have been carried out. The calculations revealed, inter alia, a novel activation free-energy pathway for HO attack on TMP in the gas phase in which initial addition at phosphorus is followed by pseudorotation and subsequent elimination with simultaneous intramolecular proton transfer. Ah initio calculations and continuum dielectric methods have been employed to map out the lowest activation free-energy profiles for the alkaline hydrolysis of a five-membered cyclic phosphate, methyl ethylene phosphate (247), its acyclic analogue, trimethyl phosphate (246), and its six-membered ring counterpart, methyl propylene phosphate (248). The rate-limiting step for the three reactions was found to be hydroxyl ion attack at the phosphorus atom of the triester. ... 

The spectroscopic, kinetic, and thermodynamic data discussed are sufficient to describe semiquantitatively the energy profile of proton transfer to a hydride ligand occurring in solution [29, 35, 36]. Figure 10.10 shows the energy as a function of the proton-hydride distance, varying from the initial state to a final product. The average structural parameters of the initial hydrides and intermediates have been taken from earlier chapters. Since proton-hydride contacts of... 

Figure 10.10 Energy profiles of proton transfer to a hydride ligand of a transition metal complex in solution AEi = + 3 to 4kcal/mol, AE2 = — 5 to — 7 kcal/mol, A 3= + 10 to 14 kcal/mol, and A 4 = —7 kcal/mol the energy is a function of the proton-hydride distance, varying from an initial state (2.5 A) to the final product (0.9 A) conversion of the intimate ion pair to the solvent-separated ion pair is shown as a function of the H+- O" distance. (Reproduced with permission from ref. 29.)...

Figure 10.12 Energy profile obtained for proton transfer from HCl via dihydrogen-bonded complex [A1H4- -HCl] along the reaction coordinate. (Reproduced with permission from ref. 38.)...

Figure 10.13 Energy profile, intermediates, and transition states (TS) obtained by the B3LYP and MP2 (in parentheses) methods for proton transfer from CF3OH to the hydridic hydrogen of the ion [BH4] . (Reproduced with permission from ref. 39.)...

Figure 10.15 Energy profile obtained by DFT calculations for proton transfer to hydridic hydrogen in hydride It from the dimer (CF3COOH)2 (TFA). Energies are given in kcal/mol. (Reproduced with permission from ref. 6.)...

As shown in Figure 2.3, the activation energy of protonation (TSlt) is only 1.9kcal/mol and the proton-transfer reaction exhibits a very flat energy profile. 

Figure 3.35 PE profiles for excited-state hydrogen transfer (full curve) and proton transfer (dashed curve) of 7HQ-(NH3)3. The calculated energies of the electronic ground state of the enol and keto forms are also indicated. The molecular orbitals contributing dominantly to the excited-state wavefunctions are shown for the minima along the hydrogen transfer and proton transfer paths. The energies have been calculated with the CIS method. (Reprinted from C. Manca, C. Tanner and S. Leutwyler, Int. Rev. Phys. Chern., 24, 457-488. Copyright (2005), with permission from Taylor Francis).

Figure 9. Energy (kcal/mol) profiles along the reaction coordinate (A) in vacuo (solid line) and in aqueous solution (dashed line) for the proton transfer reaction between H20 and [FeH(CO)4]-. The horizontal lines correspond to the energies of the products at infinite separation.

For a theoretical (ab initio) study of the potential energy profile for proton transfer from NH4 to NH3, see Delpuech et al, 1972. 

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